I hear a similar noise...I've read in other threads that some noise from the driveline is normal, especially for us M6 guys I hope they're right! Of course, after a good hard run through the gears, I always think I've hurt it! How many miles?

4200 miles on it. I took it back in and they called GM who told them it's normal for the LS3 to have sounds like that. Then they took me out in another SS they had on the lot. They said "do you hear that??" I said "no, I don't hear anything."
They were trying to convince me that the sound was in the other one too, but it wasn't.
I dono. As long as there's a paper trail when the transmission falls out of the bottom of the car, I guess I'll let it ride. I don't want to, and I shouldn't have to, but whatever.

Sounds just like a dealer! When I took mine in for "rail dust" they tried to say it was some environmental issue where I lived and I'd only had the car for 2 weeks! If you go away, their job is done. Simple. But, I hope they're right about the driveline noise because I hear it too!